The present invention relates generally to a superconductive field coil winding in a synchronous rotating machine. More particularly, the present invention relates to a rotor core that supports a superconducting field winding assembly in a synchronous machine.
Synchronous electrical machines having rotor field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more field coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, superconducting (SC) field coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known superconductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core superconductive rotors require large amounts of superconducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor.
High temperature SC rotor coil field windings are formed of superconducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K, to achieve and maintain superconductivity. The SC windings may be formed of a high temperature superconducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
High temperature superconducting (HTS) coil windings are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start up and shut down cycles over the course of several years that result in low cycle fatigue loading of the rotor. Furthermore, the HTS rotor coil windings should be capable of withstanding 25% over-speed operation during rotor balancing procedures at ambient temperature, and at occasional over-speed conditions at cryogenic temperatures during power generation operation. These over-speed conditions substantially increase the centrifugal force loading on the rotor coil windings over normal operating conditions.
SC coils used as the HTS rotor field winding of an electrical machine are subjected to stresses and strains during cool-down and normal operation. These coils are subjected to centrifugal loading, torque transmission, and transient fault conditions. To withstand the forces, stresses, strains and cyclical loading, the SC coils should be properly supported in the rotor by a coil support system. These coil support systems hold the SC coil(s) in the HTS rotor and secure the coils against the tremendous centrifugal forces due to the rotation of the rotor. Moreover, the coil support system protects the SC coils, and ensures that the coils do not prematurely crack, fatigue or otherwise break.
Developing coil support systems for HTS coil has been a difficult challenge in adapting SC coil windings to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. There is a long-felt need for a HTS rotor having a coil support system for a SC coil. The need also exists for a coil support system made with low cost and easy to fabricate components.
A multi-piece rotor core for a superconducting synchronous machine has been developed. The rotor core includes passages transverse to the rotor axis. Through these passages extend coil support bars that are coupled to a superconducting coil winding. The coil winding extends around the rotor core, and is generally in a plane that includes the rotor axis. The rotor core has flat sides that are adjacent the long sides of the coil winding.
The rotor core is assembled from several rotor core sections. These sections are generally disk shaped and have a T-shaped cross-section. The rotor core sections have connection bosses to engage slots in adjacent rotor core sections. The core sections are assembled around a pre-formed superconducting winding and coil support. The assembly of rotor core sections form a solid core, except for the support bar passages that extend through the core axis. The core sections are held together by tie rods that extend through the assembly of sections. The rods are parallel to the rotor core axis and extend the length of the core.
Tension bars that extend between the sides of the rotor coil can provide support so that the coil will withstand the centrifugal forces of the rotor. To support opposite sides of the coil, the tension bars extend through rotor core. There is a desire to assembly the tension bar and coil winding before both are mounted on a rotor core. However, a solid rotor core will not allow for pre-assembly of the coil and tension members. Thus, there is a need for a rotor core and assembly technique that will allow an assembled coil and tension member to be mounted on a solid rotor core.
An assembly of rotor core sections permits the rotor core to be assembled around a coil winding assembly. The coil winding assembly may be assembled with the winding support to form a pre-formed coil winding assembly prior to the rotor core assembly. Pre-assembly of the field coil and winding support should reduce the rotor-coil production cycle, improve coil support quality, and reduce coil assembly variations.
The HTS rotor may be for a synchronous machine originally designed to include SC coils. Alternatively, the HTS rotor may replace a copper coil rotor in an existing electrical machine, such as in a conventional generator. The rotor and its SC coils are described here in the context of a generator, but the HTS coil rotor is also suitable for use in other synchronous machines.
In a first embodiment, the invention is a rotor in a synchronous machine, comprising: a superconducting field winding assembly having a coil winding and at least one winding support extending between opposite sides of the winding, and a rotor core formed of a plurality of rotor core sections, each of said core sections having a slot to receive said winding support.
In another embodiment, the invention is a rotor core and winding assembly comprising: separable rotor core sections assembled around the winding assembly to form said rotor core, where said core sections are axially aligned with said rotor core, and said winding assembly includes a pre-assembled a superconducting field winding and a center winding support.
Another embodiment of the invention is a method for assembling a rotor core around a superconducting field coil winding assembly comprising the steps of: fabricating said field coil winding assembly by assembling a field coil winding and a coil support prior to assembly of the rotor core, inserting a portion of each of a plurality of rotor core sections partially through said coil winding assembly, assembling the plurality of rotor core sections around said coil support, and securing the assembly of rotor core sections.